Rotary systems having at least one parameter such as position, speed, torque, or acceleration which is servo-controlled as a function of time, tend to disturb the support on which they are mounted by a reaction effect. Such disturbances can generally be considered as being negligible when the support has considerable mass, e.g. a large ship. In contrast, the disturbances become significant and require compensating when the mass of the support is relatively small or when the position of the support must be maintained very accurately. This applies in particular to satellites which are required to conserve a well-defined attitude in orbit or which are subjected to the constraints of microgravity: the systems embarked on the satellite such as systems for rotating an antenna or solar panels at a non-constant speed tend to disturb the stability of the satellite unacceptably in the absence of stabilization systems.
In a paper entitled "A Reactionless Precision Pointing Actuator" given by Peter Wiktor at the "Aerospace Mechanism Symposium" held in Houston in May 1987, proposals are made to provide a control system for a gyroscopic platform which simultaneously ensures stabilization so as to prevent a reaction torque appearing on the platform support which is constituted by a spacecraft or satellite. In order to ensure decoupling between the motion of the gyroscopic platform and the attitude control of the spacecraft, counter-rotating motion is imparted to a reaction wheel incorporated in the gyroscopic platform and provided with a shaft which is coaxial with the shaft of the gyroscopic platform and which is rotated in the opposite direction thereto by means of an electric motor for the reaction wheel, having its stator mounted on the gyroscopic platform and its rotor fixed to said coaxial shaft. A direct drive second motor has its stator fixed to the support and its rotor fixed to the axis of the gyroscopic platform and serves both to drive the gyroscopic platform so as to cause it to take up an angular position or a speed of rotation as a function of a predetermined law, and also to compensate for interferring torque due to friction in the bearings or to the presence of electricity power cables which prevent angular rotations through more than 2.pi. radians. The servo-control circuit associated with the electric motor for the reaction wheel has a passband situated in a higher range of frequencies than the passband of the servo-control circuits associated with the direct drive motor.
The embodiment described in the above-mentioned paper uses two electric motors of non-negligible power, thereby increasing both mass and energy consumption, and in addition it is not suitable for applications in which the working rotary member is required to rotate through several turns.